1. Field of the Invention
This invention relates to a semiconductor device, a semiconductor device applicable for a liquid crystal display device, and a process for fabricating the semiconductor device. More particularly, the present invention relates to a semiconductor device applicable for a liquid crystal display device, comprising a non-monocrystalline semiconductor element and a monocrystalline semiconductor element which are provided on the same substrate, and a process for fabricating such a semiconductor device.
2. Related Background Art
As prior art, semiconductor devices used for liquid crystal display devices will be first described.
Liquid crystal display devices provided with active matrix elements have been hitherto made commercially available as flat panel display devices or projection televisions.
FIG. 17 schematically illustrates the construction of a drive circuit for active matrix type liquid crystal display elements that has been conventionally used. In FIG. 17, reference numeral 301 denotes a picture element switch; 305, a liquid crystal picture element; 306, a transparent substrate; 302, a buffer portion; 303, a horizontal shift register; and 304, a vertical shift register. Luminance signals and sound signals of televisions are compressed in a certain band and are transferred to the buffer portion 302 that is driven by the horizontal shift register 303 having a driving capacity enough to be able to follow up the frequencies of the band. Next, the signals are transferred to the liquid crystal in the period in which the picture element switch 301 is kept "ON" by the vertical shift register 304.
Performance required for each circuit should be considered here. Assume that, taking account of high-grade televisions, they operate at a frame frequency of 60 Hz, a scanning line number of about 1,000 lines, a horizontal scanning period of about 30 .mu.sec (effective scanning period: 27 .mu.sec) and a horizontal picture element number of abut 1,500 elements, the television signals are transferred to the buffer at a frequency of about 45 MHz. Hence, each circuit is required to have the following performance:
(1) The horizontal shift register can drive at 45 MHz or above (i.e., driving capacity); PA1 (2) the vertical shift register can drive at 500 kHz or above; PA1 (3) a transfer switch that is driven by the horizontal shift register and transfers television signals to the buffer can drive at 45 MHz or above; and PA1 (4) the picture element switch can drive at 500 kHz or above. PA1 a substrate comprising a semiconductor monocrystalline substrate on one principal surface side of which a light-transmitting film is formed; the substrate being prepared by removing from the other principal surface side thereof a semiconductor monocrystalline region present right beneath the light-transmitting film; PA1 a non-monocrystalline semiconductor element formed on the light-transmitting film; and PA1 a monocrystalline semiconductor element formed in a semiconductor monocrystalline region remaining in the substrate; PA1 the non-monocrystalline semiconductor element and the monocrystalline semiconductor element being electrically connected.
What is meant by the driving capacity herein referred to is that, when a liquid crystal picture element is made to have a certain gradation number N, a voltage not lower than the following voltage is transferred within the above period. EQU V.sub.m -(V.sub.m -V.sub.t)/N[V]
wherein V.sub.m represents a voltage that gives a maximum or minimum transmittance of a liquid crystal, and V.sub.t represents a liquid-crystal threshold voltage obtained from a V-T (voltage-transmittance) curve.
As is seen from these, the picture element switch and the vertical shift register may have a relatively small driving capacity, but the horizontal shift register and the buffer portion are required to drive at a high speed. For this reason, in existing liquid crystal display devices, measures are taken such that the picture element switch and the vertical shift register are formed in a monolithic fashion together with liquid crystals, using polycrystalline silicon or amorphous silicon thin-film transistors (TFTs) deposited on a glass substrate; in other peripheral circuits, IC chips are externally packaged. Although it is attempted to form the peripheral circuits also in a monolithic fashion using polycrystalline silicon TFTs, transistors must be made larger in size or circuits must be complicatedly designed, because of a small driving capacity of individual TFTs. Meanwhile, as for liquid crystal imaging apparatus such as VTR camera viewfinders or projection display devices, it is important for the substrate to be light-transmissive in their visible light regions.
As stated above, peripheral drive circuits with high-performance liquid. Crystal picture elements are required in order to accomplish high-performance liquid crystal display devices, and a semiconductor layer on which semiconductor elements constituting them are formed should be formed of a semiconductor monocrystalline layer having a good crystallinity. Such peripheral drive circuits must be screened from light.
As for the active matrix element which orients a liquid crystal in accordance with a signal, it need not necessarily be formed by monocrystalline transistors, but transistors must be formed on a light-transmitting film.
An example will be given below. Assume that the total load of the active matrix element is 50 fF and the voltage swing width for liquid crystal orientation is 10 V, a charge of; EQU 50.times.10.sup.-15 .times.10=5.times.10.sup.-13 (C)
must be flowed in a given time. When it is taken into account to drive this element at 500 kHz as stated above, the saturated current required for the transistors is; EQU I.sub.sat .times.1/(500.times.10.sup.3)&gt;5.times.10.sup.-13
and is; EQU I.sub.sat &gt;2.5.times.10.sup.-7 (A).
Thus, it is seen to be enough for the saturated current to be 250 nA or more. This is a value that can be well achieved by the polycrystalline silicon or amorphous silicon thin-film transistors.
Items required in the peripheral drive circuit elements and the active matrix elements can be summarized as shown in Table 1.
TABLE 1 ______________________________________ Peripheral drive circuit element Active matrix element ______________________________________ Current driving capacity: Comparable to Higher than polycrystalline monocrystalline element or amorphous element Light transmission properties of substrate: Unnecessary Necessary ______________________________________
In instances in which both the driving capacity comparable to monocrystalline elements and the light transmission properties of substrates are required in elements, a monocrystalline SOI (silicon on insulator) must be used. In the case of liquid crystal display devices, however, there is no element in which both of the above two items of performance are required, and hence one may have an idea of a constitution in which the peripheral drive circuit element and the active matrix element are separately built in.
If, however, the peripheral drive circuit element and the active matrix element are built in separate substrates, it becomes necessary to connect the both by wire bonding or the like, which results in a complicated process. Thus, it has been sought to make an improvement so that a cost decrease can be achieved and picture elements can be made much finer.